2

I want to make a really small arduino using the Atmega 328p and I noticed that it has 3 GND pins and 2 VCC pins.

I measured the pins and it seems that all GNDs are connected and all VCCs are connected to each other.

Can I rely on this feature to make my arduino schematic? Is it a good idea? Can I rely on the same idea for other components with multiple VCCs and/or GNDs?

Cheers

smiron
  • 145
  • 2
  • 6
  • 4
    No, you should connect them all, each with appropriate decoupling caps. I'm sure this has been asked at least once before, but in any case the datasheet should tell you what's required. [See this similar question for PIC devices](http://electronics.stackexchange.com/questions/22662/if-a-pic-mcu-provides-multiple-vdd-vss-should-you-provide-power-to-them-all?rq=1). – Roger Rowland Apr 29 '16 at 13:26
  • It almost sounds like you want to route your PCB using this (mis)information. If so, _don't_. What I mean is, you can't route the VCC to one pin of the chip, then continue out from the other pin to a peripheral. – pipe Apr 29 '16 at 14:53
  • That is exactly what I was intending to do :D. Thanks for clarifying. – smiron Apr 29 '16 at 18:21

3 Answers3

3

Connect all of them and more importantly apply very localized decoupling capacitors on them all.

Andy aka
  • 434,556
  • 28
  • 351
  • 777
2

Generally, a device with multiple supply/ground pins intends for those pins to be externally connected to the appropriate supply/ground. It is particularly important to connect the ground pins together using wide traces or polygons/planes to avoid ground loops. The datasheet, section 1.1, for the 328p indicates three specific power connection types: VCC, GND, and AVCC. Thus, all of the VCC pins should be connected together, the GND pins should be connected together with care for a low impedance connection, and the AVCC pin should be connected (with optional filter described in the datasheet) to VCC.

While this device shows a direct connection between supply/ground pins, some devices require connections between those pins externally and may be damaged if some pins are left floating. It is best to assume that pins of the same supply must be connected together unless specifically noted in the device datasheet. Multiple external connections allows more current to safely flow into/out of the device.

Additionally, considering decoupling: multiple supply pins allows multiple bypass capacitors (usually 1000pF to 0.1uF, depending on the application) to be placed around the device. For a low speed device with short connections between supply pins and an uninterrupted ground plane around the device, a single 0.1uF capacitor may suffice. Higher speed devices may require one capacitor per supply pin or even multiple sizes of capacitor on each supply pin.

user2943160
  • 2,878
  • 1
  • 17
  • 32
2

Connect them all.

The reason why multiple pins of the same potential are brought out of the die is for two reasons.

1) this reduces the resistance. When you do that you reduce the voltage bounce that occurs from current pulses that are formed upon switching. So you get a cleaner voltage source on the internal power rail.

2) it reduces the inductance of the wire bond leads from the bonding shelf to the die. Reducing the inductance makes any decoupling capacitance more effective and therefore reduces any internal rail bounce resulting in again, cleaner internal power voltages.

placeholder
  • 29,982
  • 10
  • 63
  • 110